Watercraft are traditionally propelled by paddles, paddle wheels, propellers, impellers, and sails. Paddles are a wonderful technology for driving a thrust fluid and propelling a watercraft. While simple and versatile, paddles are relatively inefficient when compared to propellers. Among the non-sail propulsion types, propellers generally exhibit the greatest efficiency (in terms of converting input work into output thrust) at approximately 60%, though generally only if significant effort is taken to match the propeller to the boat's displacement, average speed, placement of the engine, angle of the drive-shaft, and other parameters. For most propeller driven watercraft, overall efficiency at the propeller is significantly less, on the order of 40%. In addition, the efficiency curve of a propeller generally follows the form of an inverted parabola, with peak efficiency achieved at a narrow range of speeds at the top of the efficiency curve.
In addition, design of propeller driven watercraft involves a number of well known compromises involving propeller size, placement of the engine relative to the propeller location, and hull shape, to name but a few of the issues. In addition, the thrust fluid propelled by a single propeller rotates. Rotation of the thrust fluid does not produce thrust, though is required in order to move the thrust fluid backward (which does produce thrust). Thrust fluid rotation can be eliminated or at least balanced through the use of two counter-rotating propellers, though this results in twice the propeller skin area, usually additional propeller frontal area, and (typically) twice as much drive train complexity, which reduces efficiency and decreases reliability.
Fish and marine mammals propel themselves with fins. Fins on aquatic creates exhibit greater efficiency, on the order of 80%. In addition, the efficiency curve is flatter, with very high efficiency achieved across a range of speeds, from slow to fast.
Attempts have been made to propel human-transporting watercraft with fins, though connecting the motor (be it a human or another motor) to the fins and matching the hull to the propulsion system has proven to be challenging. Connecting a motor to a fin is a complex problem, particularly in a marine environment and particularly in the context of a human power source.